1. Field of Invention
The present invention relates to plasma processing and more particularly to a method for improved plasma processing.
2. Description of Related Art
Typically, during materials processing, plasma is employed to facilitate the addition and removal of material films when fabricating composite material structures. For example, in semiconductor processing, a (dry) plasma etch process is utilized to remove or etch material along fine lines or within vias or contacts patterned on a silicon substrate. The plasma etch process generally involves positioning a semiconductor substrate with an overlying patterned, protective layer, for example a photoresist layer, into a processing chamber. Once the substrate is positioned within the chamber, an ionizable, dissociative gas mixture is introduced within the chamber at a pre-specified flow rate, while a vacuum pump is throttled to achieve an ambient process pressure.
Thereafter, a plasma is formed when a fraction of the gas species present are ionized by electrons heated via the transfer of radio frequency (RF) power either inductively or capacitively, or microwave power using, for example, electron cyclotron resonance (ECR). Moreover, the heated electrons serve to dissociate some species of the ambient gas species and create reactant specie(s) suitable for the exposed surface etch chemistry. Once the plasma is formed, any exposed surfaces of the substrate are etched by the plasma. The process is adjusted to achieve optimal conditions, including an appropriate concentration of desirable reactant and ion populations to etch various features (e.g., trenches, vias, contacts, etc.) in the exposed regions of the substrate. Such substrate materials where etching is required include silicon dioxide (SiO2), poly-silicon and silicon nitride.
As the feature size shrinks and the number and complexity of the etch process steps used during integrated circuit (IC) fabrication escalate, the ability to control the transport of reactive materials to and effluent etch products from etch features, in order to achieve the proper chemical balance necessary to attain high etch rates with good material selectivity, becomes more stringent.
The etch rate in most dry etch applications, for example oxide (SiO2) etch, includes a physical component and a chemical component. The plasma chemistry creates a population of positively charged (relatively heavy) ions (such as singly charged argon ions) utilized for the physical component and a population of chemical radicals (such as atomic fluorine F, and CF, CF2, CF3 or more generally CFx species in a fluorocarbon plasma) utilized for the chemical component. Moreover, the chemical reactants (CFx) act as reactants in the surface etch chemistry and the (heavy) positively charged ions (Ar+) provide energy to catalyze the surface reactions.
As feature sizes progressively shrink, they do so at a rate greater than a rate at which the oxide (and other film) thicknesses shrink. Therefore, the etch feature aspect ratio (feature depth-to-width) is greatly increased with shrinking sizes (of order 10:1). As the aspect ratio increases, the directionality of chemical reactant and ion transport local to the etch features becomes increasingly important in order to preserve the anisotropy of the etch feature.
The transport of electrically charged species (such as ions) can be affected by an electric force and, therefore, it is conventional in the industry to provide a substrate holder (chuck) with a RF bias to attract and accelerate ions to the substrate surface through the plasma sheath such that they arrive moving in a direction substantially normal to the substrate surface. However, the transport of neutral, chemically reactive species is not amenable to the application of an electric force to assert their directionality at the substrate surface.